Thermosensitive resistance element



Jan. 13, 1959 G. N. HOWATT THERMOSENSITIVE RESISTANCE ELEMENT Filed Nov. 25, 1957 FIG. 2

/NVEN7OR GLENN N. HOWA 77' WMAW AGENT United States Patent;

2,358,935 Eatented Jan. 13, 1959 ice THERMOSEN SHTIV E RESISTANCE ELEMENT Glenn N. Howatt, Metuchen, N. 1., assignor to Quite-n Industries, Inc., Metuchen, N. L, a corporation of New Jersey Application November 25, 1957, Serial No. $8,413

12 Ciairns. (Ci. Mil-63) This invention relatesin general to thin-film resistance "devices, and more particularly to devices of the type named which are temperature sensitive, and which are known in the art as thermistors.

In the design of thermistors and thermosensitive elements, characteristics which are of importance include the voltage-current response and the resultant negative temperature coefficient of resistance over the temperature range of interest, and also the response-time required for the thermistor body to heat up to its ultimate temperature. The former characteristics are, in general, inherent in the material of which the thermistor is fabricated. However, the time-response of a thermistor element is largely a function of the mass between the electrodes, since the smaller the mass, the more rapidly it will be heated by the passage of current, and also, by external heat sources, and the shorter its time-response will be. A thermistor characterized by a short time-response is of considerable value in numerous types of applications, such as, for example, in alarm systems, in the operation of relays, in temperature-control circuits, infra-red detectors, etc.

In order to provide thermistor elements having shorter time-responses, so-called flake thermistors have been designed having thicknesses of only a few microns, between electrodes. However, because of their small dimensions and fragile construction, numerous problems have arisen in connection with the design, fabrication, and handling of flake or thin-film thermistors. More over, the connecting leads normally attached to flake thermistor bodies which are at least a mil, or a half mil in cross-section, function to rapidly conduct away the heat from the thermistor body, to some extent reducing the rapid time-response. w

it is, therefore, a general object of the present invention to improve the construction and fabrication of thinfilm resistance elements, and more particularly thermisto-rs.

A more specific object of the invention is to provide thermistors having a very short time-response, which are nevertheless, less fragile and more readily adopted for circuit applications than the flake thermistors of the prior art.

These and other objects are attained in a resistance or thermistor structure in accordance with the present invention which comprises a thin ceramic wafer containing an opening which is covered over with a thin film of the order of between one and 50 microns thick,

points on the upper and lower portions of the outer periphery of the ceramic wafer, to provide external connecting points.

A preferred method, which will be described in greater detail hereinafter, for fabricating a thin film thermistor of the present invention, involves grinding the temperature-sensitive resistance material which may comprise, in accordance with one specific example, metallic oxides of manganese, nickel or cobalt. These are ground to a final degree of fineness. The powder, so obtained, is combined with a thermoplastic binder, such as a solution of ethylcellulose, and further milled, solvent being added, as needed, to obtain the desired degree of fluidity, which is a determining factor in the thickness of the film. In the case of a cellulose binder, a small amount of castor oil is added to serve as a plasticizer. The dispersion, so formed, is then dropped on the surface of the water, at room temperature, where it forms a thin floating film the spread of which is limited by some simple means.

A tiny ceramic wafer having a central opening is then brought up under the floating film, lifting it off of the surface of the water. The ceramic wafer, together with the superposed film, is then baked to maturity, and electrode coatings are vacuum-evaporated onto the opposite surfaces thereof through masks designed to provide the desired pattern.

Alternatively, a film, sufliciently thin for the purposes of the present invention, can be formed by the method described in detail in my Patent 2,486,410, issued November l, 1949.

A particular feature of a thermistor element formed in accordance with the teachings of my present invention formed from powdered crystalline material having a high is that, although the film sandwiched between the elec trodes is very thin, providing a rapid time-response, the thermistor is nevertheless, simpler to fabricate and to handle than prior-art thin-film resistance elements, because of the supporting ceramic structure.

As an additional feature, instead of the wire connecting leads which are ordinarily attached to flake thermistors and which tend to conduct the heat away from the unit, thermistors constructed in accordance with the present invention may employ much thinner leads, which are evaporated or otherwise deposited onto the supporting ceramic structure.

These, and other objects, features and advantages, will be readily apparent to those skilled in the art from a study of the detailed specification hereinafter, and the attached drawings, in which:

Figure 1 shows a step in a preferred process for fabricating a resistance or thermistor element in accordance with the present invention, by forming a thin film on a wafer surface;

Figure 2 shows a thin ceramic wafer containing a hole, the wafer forming the supporting body of the resistance of thermistor element of the present invention;

Figure 3 is a perspective showing of another step in the process of the present invention, in which the wafer of Figure 2 is placed beneath the floating film, to lift the film off the water surface;'

Figure 4 shows the thin film, cured on the surface of the ceramic wafer, and a pair of electrodes and connecting leads evaporated thereon;

Figure 5 is a cross-sectional View of the structure of Figure 4; and

Figure 6 is a cross-sectional view of a modified form of the structure of the present invention employing a pair of wafers.

A preferred process of fabricating a thin film of thermosensitive resistance material in accordance with the present invention will be described in detail as follows: A powder of material, which, after firing or curing, has a high temperature coefficient of resistance is ground to a Mix 1: Percent (by weight) Manganese dioxide, MnO 80.1 Nickel oxide, Ni 17.6 Cobalt oxide, CoO 2.3

Mix ll fporcelain, for high temperature applications] Percent (by weight) Silicon dioxide, SiO O Feldspar Clay Balance Mix Ill [for high temperature applications]:

Percent (by weight) Manganese dioxide, MnO 100 Mix 1V: Percent (by weigh) Iron oxide, Fe O 90 Sodium tantalate, NaTaO 10 Mix V: Percent (by weight) Iron oxide, Fe O 90 Sodium nio'oate, NaNbO 10 Additional examples of mixes useful for the him of the present invention include compositions which vary from 100 percent by weight of iron oxide, Fe O through all the intermediate combinations to three parts iron oxide and one part titania. Still further examples are compositions consisting of 50 percent by weight of iron oxide and another 50 percent, which may be entirely of cobalt oxide, or of nickel oxide, or any combination of the two; or as another alternative, the foregoing combinations of the oxides of iron, nickel, and cobalt, to which up to percent by weight of copper oxide has been added.

After the thermistor material has been ground in the manner indicated, a thermoplastic organic binder is added, such as, for example, a 2 percent by volume solution of ethylcellulose in toluene, or a 5 percent by volume solution of chlorinated rubber, in toluene, or alternatively, any of a number of additional organic binders consisting of resins and plastics, together with solvents and adjusting media, of the types and in the formulations disclosed in my Patent 2,582,993, issued January 22, 1952, are suitable for the purposes of the present invention. As disclosed in that patent, for example, the proportion of the binder-solvent system to the solid matter may be, for example, a ratio of about 20 percent to 80 percent by weight of solid inorganic matter.

In the present specific illustrative example, Mix 1, after it has been ground to the final degree of fineness, is combined with a 10 percent by volume solution of ethylcellulose. This combination is then again mixed in a ball-mill until it becomes substantially homogeneous, further toluene or similar solvents, being added, as necessary to produce a dispersion having a fluidity within the range one to five-hundred poises. This may be plasticized, for example, by the addition of Mt percent, by volume, of castor oil. The dispersion is further milled until it again becomes homogeneous.

The dispersion is then dropped on the surface of water maintained at room temperature in an ordinary shallow container 1 such as shown in Figure l of the drawings. There, it forms a thin film 2, which is supported on the surface of the water 3. A ring 4 of, for example, plastic or metal, and having a diameter of about Me inch, is placed in the container 1 to control the spread of the film 2. It will be apparent to those skilled in the art, that any other liquid which has sufficient surface tension to support the film 2, and which is chemically inert to the film, may be used in place of water.

In the manner indicated, films within the range one to fifty microns in thickness, can be formed.

Figure 2 of the drawings shows in rectangular wafer a, about /2 inch, by inch and about 15 mils thick, and having a centrally located hole 5 about inch in .meter.

accordance with an illustrative embodiment of the present invention the wafer 6 is formed of steatite or Zirconia porcelain ceramics. The composition of the binder for the wafer 6, and the manner of processing is disclosed in detail in my Patent 2,486,410, supra. For the purposes of the present invention, a wafer so formed is preferably, simply dried, and cut to the desired shape without firing; however, alternatively it may be fired in advance of the herein disclosed process. The water so formed has a coefficient of thermal expansion per degree centigrade within the range 4 to l5 10, which is matched to that of the film 2, to within about 0.5 X 10 Likewise, the wafer should exhibit a linear shrinkage, upon firing, of between about 8 and 30 percent, which is matched, within about /2 percent to that of the film 2.

Alternatively, the wafer 6 can be formed of any equivalent ceramic, or crystalline material, or from any vitreous insulating material having a coefficient of thermal expansion which substantially matches that of the film 2 up thr the firing or curing temperature, and which does not melt or soften at temperatures up to the upper limit of this range.

As indicated in Figure 3, the wafer 6 is slid underneath the film 2. resting on the surface of the water, and is lifted gently, so that the film 2 is not broken, and so that it covers most of the flat upper surface of the wafer 6, including the central hole 5.

wafer 6, with the film 2 resting on top, is then bak d until the film matures, that is, until all the water vapor and volatile nts are driven from it, leaving it tough, flexible, and leathery.

in the present illustrative embodiment, using Mix l in combir ion with the disclosed hinder, the best firing temperature has been found to be substantially about 2360 degrees centigrade.

film-covered Wafer o is heated up fr m room temperature in about two hours, maintained the peak temperature of about 230-) degrees Fahrenheit (i266 degrees centigrade), for ent a half hour, and removed from. the oven and allowed to cool to room temperature in about two hours. During firing process, the film 2 bonds firmly to the surface the wafer 6.

in accordance with a further step in the fabrication of the device, electrode contacts 7, and it), cnd their respec tive leads and termin 13, 8, 9 and 11, 1'2 are evaporated onto the upper and lower surfaces of the film 2 in combination With the wafer 6 in the manner shown in perspective in Figure 4, and in cross-section in Figure 5,

" and 19 are of any highly conducting The electrodes 1 material. which forms a thin, even coating, which is che nically inert to the ceramic or the thermistor film, such 6X81; ie, sii er, platinum, or gold, or certain alloys of se and other metais which ma serve to increase their conducting and spreading qualities.

In the actual example under description, a pair of which can readily be cut to provide openings of the desired e, an is impervious to metal vapor, are carefully place. on the upper and lower surfaces of the film are provided with small openings, centrally located, of about to of an inch in diameter, adjacent to the upper and lower surfaces of the film. Extend ring from each of these central openings to the edges of the ceramic wafer 6, on its upper and lower surfaces, is a thin opening about the width of a pencil line, terminatmasks ing in a larger opening adjacent to the perip'ieral propon tion of the wafer. The assemblage, including the masks, is placed in an evacuator cha'nber, wherein silver, or one of the equ alent metals or combination of tals mentioned pre usly, is evaporated onto both the upper lower surfaces or" the device to a thickness of, for e: ample, several hundred angstroms. The mask is then removed, leaving electrodes 7 and llfi on the upper and lower surfaces of film 2., and the respective conducting leads 8, and lit, connected to respective t -minal points 9 and 12 on the upper and lower periphei.-. the wafer 15. The resultant pattern of the electrodes and leads is shown in perspective in Figure 4, and in cross section in Figure 5.

It will be apparent that the concept of the present invention is not limited to the specific form disclosed Numerous variations and alternative configurations be apparent to those skilled in the art, such as, for er:- ample, the structure illustrated in Figure 6, in which the film is sandwiched between a pair of wafers each having matched openings, the assemblage being fired as a unit. In Figure 6, the matched supporting wafers are designated 6a and 6b, and the other reference characters correspond to those of Figure 5.

Moreover, the electrodes are not necessarily positioned opposite one another on the film 2, but their locations may be varied in order to vary the resistance of the thermistor element.

Further, the thickness of the electrodes are not to be construed as limited to any specific range, but may vary from as little as a few hundred angstroms to a mil or more, depending on the conductivities and spreading qualities of the metallic components employed.

What I claim is:

l. A resistance element comprising in combination a thin insulating wafer, containing an opening of substantial size, a thin heat-cured film formed from a powdered insulating material characterized after firing by a high temperature coetficient of resistance, suspended in a thermoplastic binder, said film covering said opening and thermally bonded to at least a portion of one surface of said water, and electrodes coupled to opposite surfaces of said film.

2. A thermosensitive element comprising in combination a ceramic wafer containing an opening of substantial size, a thin heat-cured film formed of a powdered material characterized after firing by a high temperature coefiicient of resistance, suspended in a thermoplastic binder, said film covering said opening and thermally bonded to at least a portion of one surface of said wafer, and a pair of electrodes disposed at separated positions on said film.

3. A combination in accordance with claim 2 wherein said ceramic and said film have substantially the same temperature coefficients of thermal expansion and the same linear shrinkage over the range from room temperature up to an including the maturing temperature of said film.

4. A combination in accordance with claim 2 wherein said wafer is a ceramic consisting essentially of zirconia porcelain.

5. A combination in accordance with claim 2 which includes a pair of terminals fixed at peripheral positions on opposite surfaces of said wafer, and leads comprising thin films of conducting material connecting said terminals and said electrodes.

6. A combination in accordance with claim 2 wherein said powdered material consists of substantially about 80.1 percent of manganese oxide, 17.6 percent of nickel oxide and 2.3 percent of cobalt oxide, all by weight, in a cellulose binder.

7. A thermoscn itive element c: '1 a ceramic .ier of the order of taining an open' 5 of subst ntiai film of a powdc c. firing by a l temperature co lcient of resistance, suspended in er3; lastic b ier, said film cc" ering said opening and thermally bonded to at least a portion of one surface of said wafer, and a pair of trodes comprising evap ed layers of conducting material coated es of said filn wherein said ceramic have subsiantially the same coefiicient "*d the same linear shrinkage over mperature up to and including the the said film. isnive element comprising in combinaa ramic wafers substantially similar size and shape, each of the order of about 15 mils thick, eac' containing matched openings of substantial size, a thin heat-cured of about between one and 50 microns thick formed of a powdered crystalline material characterized after firing by a high temperature coefiicient of resistance, suspended in a thermoplastic binder, said film sandwiched between said wafers covering said matched openings and thermally bonded between contiguous surfaces of said wafers, the assemblage including said pair of wafers and said film sandwiched therebetween having been fired together, and a pair of electrodes in contact with opposite surfaces of said film, wherein said ceramic war and said film have substantially the same coefficients of linear expansion and the same linear shrinkage over the range from room temperature up to and including the maturing temperature of said film.

9. The method of forming a thermosensitive thin-flake resistance element which comprises the steps of grinding crystalline material to a fine powder, said material characterized upon firing by a high temperature coeificient of resistance, mixing the said powder with a thermoplastic binder, adding solvent to the said mixture to obtain a dispersion having a fluidity of between one and about five-hundred pcises, adding castor oil to the amount of less than 1 percent by Weight of the binder to said dispersion, dropping small quantities of said dispersion on a liquid surface chemically inert thereto, whereby said drop spreads out to form a thin film, limiting the spread of said film, employing a thin ceramic wafer containing a substantial hole, having a coelficient of thermal expansion and linear shrinkage similar to that of said film,

mm from said liquid surface, baking said ceic combinaick, coneat-cured rmed ed alter to lift said ramic water, including said film covering said hole and a substantial portion of said surface, to the maturing temperature of said fi m, and slowly cooling said ceramic water including said film to room temperature.

10. The method in accordance with claim 9 wherein contacting means are evaporated to spots on the opposite surfaces of said film.

11. The method in accordance with claim 10 wherein said contacting means are evaporated on opposite surfaces of said film in vacuo through a pair of masks shaped in accordance with the selected electrode pattern.

12. The method in accordance with claim 9 which includes the additional step of superposing on the assemblage including said first wafer and said film an additional ceramic wafer, matched in size and shape to said first ceramic wafer and containing a hole in matched position, and baking to maturity the assemblage including said two wafers and said film sandwiched therebetween.

No reference cited. 

1. A RESISTANCE ELEMENT COMPRISING IN COMBINATION A THIN INSULATING WAFER, CONTAINING AN OPENING OF SUBSTANTIAL SIZE, A THIN HEAT-CURED FILM FORMED A POWDERED INSULATING MATERIAL CHARACTERIZED AFTER FIRING BY A HIGH TEMPERATURE COEFFICIENT OF RESISTANCE, SUSPENDED IN A THERMOPLASTIC BINDER, SAID FILM COVERING SAID OPENING AND THERMALLY BONDED TO AT LEAST A PORTION OF ONE SURFACE OF SAID WAFER, AND ELECTRODES COUPLED TO OPPOSITE SURFACES OF SAID FILM. 